This invention relates to an electrographic developing method, and more particularly to a method of developing electrostatic latent images to visible images with a magnetic brush formed of a developer.
Electrophotographic processes generally include the steps of uniformly charging a photosensitive member and projecting an optical image onto the charged surface of the photosensitive member to form on the photosensitive surface an electrostatic latent image corresponding to the optical image. The theory of latent image formation is well known; when exposed to the optical image, the photosensitive member becomes conductive where the light impinges thereon, permitting the surface charges given by the charging to dissipate or decay in the conductive areas.
The photosensitive surface bearing the resulting electrostatic latent image involves potential differences between the exposed areas (with little or no charges) and the unexposed areas (charge retaining areas). Due to the presence of such potential differences, the latent image on the photosensitive surface can be developed to a visible image with fine pigmented toner particles electrostatically so charged as to be attractable to the unexposed areas (or exposed areas for reversal development) when the toner is applied to the photosensitive surface.
Various methods of developing electrostatic latent images in this way have been proposed and introduced into use. Typical of such methods is so-called "magnetic brush development" in which the surface of the latent image-bearing member (e.g. the above-mentioned photosensitive member) is brushed with a magnetic brush formed of a developer. This developing method usually employs a two-component developer composed of a carrier of iron granules about 100 to about 200 .mu.m in diameter and a pigmented toner about 10 .mu.m in particle size. The carrier material and the toner material are so selected that when the two materials are mixed together, each material becomes triboelectrically charged to a polarity opposite to that of the other and that the particulate toner material usually has a polarity opposite to that of the electrostatic latent image when so charged. When developing, the developer containing the toner particles electrostatically clinging to the surfaces of the carrier granules is brought into brushing contact with the latent image-bearing surface, whereupon the toner particles alone are separated from the carrier granules by the combined action of mechanical and electrostatic forces and deposited on the latent image areas by the charges of the image, thus developing the latent image to a visible image.
Although the developing method described is very useful and has many advantages, the method still has the following problems. (1) Unless the toner particles and the carrier granules are maintained in a constant mixing ratio at all times, the developer fails to produce the desired effect with stability. Moreover difficulties are encountered in maintaining the constant mixing ratio because of the limited range of permissible ratios. (2) When the developer is used for a prolonged period of time, so-called spent toner, which is no longer useful for development, becomes fused to the surfaces of carrier granules, thus degrading the developer and consequently giving toner images of reduced quality. This necessitates periodic replacement of the developer which is cumbersome and uneconomical. (3) The developed images have a narrow latitude, are not free of fogging and have a low degree of resolution.